In order to clean a solid surface, for example, to preserve metal against deterioration, remove graffiti from stone or simply to degrease or remove dirt from a solid surface, it has become common practice to use an abrasive blasting technique wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Such abrasive blasting has been used favorably, for example, to degrease metal and has replaced the environmentally unsafe solvent treatments.
Various abrasive blasting techniques have been used including dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, and a process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage.
The blast media or abrasive particles most widely used for blasting surfaces either by dry or wet blasting to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e., silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to sand as a blast media, particularly, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, stainless steel, composite surfaces, plastics, ceramic tile, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from metal surfaces and as well remove coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subsequent to blasting can be simply washed away without yielding environmental harm. Since sodium bicarbonate is water soluble and is benign to the environment, this particular blast media has found increasing use in removing coatings and cleaning dirt, grease and oil and the like from hard surfaces such as steel and interior surfaces which contact food such as in environments of food processing or handling.
Wet blasting to remove coatings and other contaminants from solid surfaces using a high pressure water stream either alone, or preferably, in conjunction with an abrasive blast media has advantages of economy over utilizing a dry blasting technique in which compressed air is used as the carrier for the blast media from the nozzle structure to the targeted surface. It is relatively easy to pump the water carrier to the high pressures utilized in the wet blasting technique. On the other hand, the compression equipment needed to compress air to even the modest pressures utilized in the dry blasting technique is quite expensive.
In order to optimize the efficiency of wet blasting for removing a coating or other contaminants from a targeted surface, it is important that the high pressure liquid stream contacting the surface remain coherent from the time the stream exits the blast nozzle to the time the stream contacts the target surface. What is meant by the term coherent is that the stream does not disperse prior to contacting the targeted surface. Dispersement of the water stream reduces the efficiency of the stripping operation since the stripping efficiency is dependent upon the mass and velocity of the high pressure liquid stream at the point of contact with the target surface. Unfortunately, in wet blasting operations, there occurs a dispersal or atomization of the liquid stream prior to contact with the targeted surface. Moreover, the atomized liquid creates a mist and can create visibility problems similar to dust formation during dry blasting operations. Poor visibility can cause the operator of the blast nozzle to continuously overlap portions of the targeted surface already stripped by contact with the high pressure liquid stream. The mist can also react unfavorably to the surrounding environment, for example create moisture problems on nearby surfaces or equipment.
It is believed that one reason for the atomization of the liquid stream as it exits the blast nozzle and prior to contact with the targeted surface is that there is a substantial amount of gas in the liquid stream. This gas escapes during travel of the stream to the targeted surface causing atomization of the liquid stream.
Dissolved gases such as air in liquid streams may also adversely affect pump efficiency, in particular, when the liquid stream is being pumped to high pressure, i.e. 1,000 psi +, such as by positive displacement piston pumps. High pressure water streams have use in blast cleaning, cutting and drilling applications where water pressures as high as 20,000 psi are useful. The dissolved gases result in cavitation, especially on the inlet stroke of the pump. Cavitation reduces pump output volume, increases power consumption and causes wear on moving parts.
Accordingly, the primary objective of the present invention is to wet blast a targeted surface for the removal of coatings or other contaminants therefrom and to maintain the abrasive liquid stream as a coherent stream as it contacts the targeted surface so as to improve stripping efficiency.
Another object of the present invention is to provide a wet blasting process to remove coatings and other contaminants from a surface and to eliminate the atomization of the high pressure liquid stream prior to contact with the targeted surface.
Still another object of the invention is to improve pump efficiency especially when pumping liquids to high pressure.
Yet another object of the present invention is to improve pump efficiency when pumping liquids and slurries.